Field of the Invention
The present invention relates to offshore gas/liquid contact columns, and more particularly to offshore gas treatment, CO2 capture, dehydration or distillation units.
Description of the Prior Art
Offshore gas treatment and/or CO2 capture units using amine wash processes comprise liquid or gaseous fluid absorption and regeneration columns. These columns operate under counter-current or co-current gas/liquid flow conditions and they are, for example, installed on vessels, floating barges or offshore platforms, of FPSO (Floating Production, Storage and Offloading) type or of FLNG (Floating Liquefied Natural Gas). Floating barges also comprise distillation columns or dehydration columns.
The columns used in these offshore gas treatment and/or CO2 capture and/or distillation and/or dehydration units are generally based on the principle of a material and/or heat exchange between the gas and the fluid that circulate in the columns. FIG. 1 shows a particular case of a gas treatment column (1) equipped with a distributor tray at the column top. Conventionally, this gas treatment column (1) comprises several sections (3) filled by a contactor. A distributor tray (2) is arranged above each section (3). The gas/liquid contactor contacts gas (G) with liquid (L) to allow exchanges.
The standard distributors (2) used in absorption/regeneration or distillation columns generally include a collector/distributor tray equipped with chimneys (4) (see FIG. 2). Distribution of the liquid occurs through passage thereof in orifices (5) positioned in the lower part of tray (2) and distribution of the gas occurs through chimneys (4). Each chimney (4) allows passage of the gas, according to the counter-current or co-current operating mode, from the lower part of the column to the upper part of column (1), or from the upper part to the lower part. Chimneys (4) project beyond one side of tray (2) and they are perpendicular thereto. Each chimney (4) has several walls, which for example are parallelepipedic or cylindrical, which define an inner volume that is open on either side of tray (2). In order to prevent the liquid from passing through the chimneys (4), the gas outlet or inlet opening above the tray (according to the counter-current or co-current mode) is preferably orthogonal to the longitudinal direction of chimney (4). The purpose of the distributor tray is to distribute liquid (L) homogeneously onto gas/liquid contactor (3).
The trays equipped with chimneys can be of different types and positioned according to different configurations. Different distributor tray variants are described in the following U.S. Pat. Nos. 6,338,774 B, 6,149,136 and 5,752,538 and U.S. Published Application 2004/020238.
The gas/liquid contact columns are placed on floating structures, of vessels, platforms or barges which are sensitive to wave motion. The equipments installed on these units, notably gas/liquid distributor trays, undergo wave motions up to six degrees of freedom (yaw, pitch, roll, heave, sway, thrust).
By way of example, the angle associated with the combination of the pitch and roll oscillations is of the order of +/−5° with a period from 15 to 20 s. The orders of magnitude of the longitudinal, transverse and vertical accelerations encountered in the column range respectively between 0.2/0.7/0.2 m/s2 6 m above the deck where the column is arranged and 0.3/1.2/0.3 m/s2 50 m above the deck.
Under such conditions, the operation of conventional distributor trays equipped with chimneys (FIG. 2) can be greatly disturbed. Indeed, the operation of these distributors is mainly based on gravity, and a liquid guard level of homogeneous height “h” has to form on the distributor tray. The square of the velocity of flow of the liquid through orifices (5) in the lower part of tray (2) is proportional to the height of the liquid guard level (UL2 ∝gh). When tray (2) is inclined under the effect of the wave motion (FIG. 3), the height of the liquid level is no longer uniform on the distributor tray (h1>h2), which causes an imbalance in the distribution of liquid at the inlet of gas/liquid contactor (3). The distribution quality and thus the efficiency of the column are greatly impacted. This poor distribution, if it is not controlled, can substantially degrade the performances of the column. A great liquid guard height (around 0.6 m) would be necessary to make up for these effects, which means bulk and weight increase, which is not suitable for offshore units.
In order to avoid this type of problem, distribution elements relatively insensitive to horizontal variability have been used. These distributors generally have a collector and a distributor connected by one or more relatively long vertical lines so that the distributor remains on charge whatever the wave motion conditions that are encountered. These distributors are generally insensitive to the effects of the wave motion and they generate a good distribution quality, but they are very bulky. They can be several meters high in some cases as described in U.S. Published Application 2004/020238.
Another solution to these problems is described in French Patents 2,771,018 and FR 2,771,019. They use two distributors (primary and secondary). Each distributor is divided into compartments in which the liquid spreads. These compartments allow the liquid to be better distributed in case of inclination of the column. However, this option remains cumbersome because it requires two distributors. Moreover, the compartments do not communicate with one another, so the liquid is not evenly distributed in the compartments.